Honestly this might be seen as rude or blunt, but let me just say this; leave the management of the server to the administrators and internal affairs team, and you guys moderate. Seriously.
It is not my decision that we ban people who are underage, it's literally a partner guideline that this server must follow, and unbanning him just breaks that. I know it's lame, playing by the book, but it's how things are.
I swear I see a conversation about how dumb it is that we ban people who are underage, or even joke about it, but this is Discord's guidelines; not some personal ones I made up. If you jokingly told a Discord employee in DMs that you were underage, you'd get banned on the spot, I know people that this has happened to as well (and I mean on the spot literally).
If you have a problem with this rule, email Discord, have them resolve it or lift it; I'll give you a hint though, they won't lift it, because it's illegal for Discord to store data of a minor (someone under 13). So why does Discord take stuff so seriously, even jokes? Because well.. they like don't want to break the law..?